Add note on stack pointer override in Chapter 4

[achester88]

After the switch to using the cpu VRAM the stack pointer retrieval from 0xFFFC no longer works. This is fixed in chapter 4 in main.rs by setting the stack pointer to 0x0600 between reset and run sections. link

When I ran into the issue I thought I had made a mistake with the bus addressing and debugged for quite some time before realizing that the read and write were just ignored and the stack pointer manually set.

This definitely feels like something that should be mentioned in the book.